Schematic diagram depicting a mid-ocean ridge hydrothermal vent site and potential microbial habitats in the subseafloor. Seawater is cycling through the seafloor where it is geothermally altered. Hot, reducing fluids containing mM concentrations of S2- ascend to the seafloor where they are either exiting undiluted through black smokers or are mixing in varying proportions with seawater in the subseafloor before exiting the seafloor at diffuse-flow vent sites. The latter creates a range of physicochemical conditions and energy sources that can be exploited by different types of microbes living in the subseafloor. The stylized cell depicts a chemolithoautotrophic sulfur-oxidizing bacterium that can use oxygen or nitrate as an electron acceptor. The growth of these organisms in the subseafloor is primarily expected to occur at temperatures between 4 and 50˚C. (Jack Cook, WHOI Graphic Services)